B-Lactam Antibiotics X H X H RCONH- A COOH COOH X=-H,-OCHg X=-H,-OCH3 Carbapenem Penicillins Cephalosporins NH Penem Oxypenam Monobactamβ–Lactam Antibiotics N S O X H RCONH COOH X= -H, -OCH3 Penicillins N O X H RCONH S COOH A X= -H, -OCH3 Cephalosporins N O Monobactam NH O Carbapenem N O S Penem N O O Oxypenam